The Study Of Monochloramine Decay In Drinking Water Distribution System

Chlorine disinfectant decay in drinking water distribution system is an essential point for the study of biological stability and chemical stability and disinfection by products which are three hot issues in the study of water quality in drinking water distribution system. Monochloramine bulk decay under the effect of various factors was studied through the experiment in laboratory. It indicates that organic matters take part in the reactions with monochloramine and can accelerate monochloramine decay. During the process of monochloramine decay, COD_Mn,TOC and UV₂₅₄ maintains at the same concentration basically. It demonstrates that monochloramine can not oxidize organic matters into CO2 because of its low oxidation potential and the conjugate bonds of organic matters has not been damaged in the process of monochloramine decay ; Nitrite could accelerate the decay of monochloramine and it can react with monochloramine quickly; Ammonia has no effect on the bulk decay of monochloramine on the whole; Hydrogen ion can catalyze the process of monochloramine auto-decompose, so low pH could accelerate monochloramine bulk decay; The acceleration effect of higher temperature on the reaction between monochloramine and organic matters is grater than it is to monochloramine auto-decompose within the range of 3℃²5℃; Monochloramine bulk decay coefficient has negative relationship with monochloramine initial concentration .On the basis of analysis to the mathematics relation between monochloramine bulk decay coefficient and various factors, predictive model of bulk decay coefficient was build and validated and the model has good result.The decay of monochloramine under the effect of bio-film incubated in annular reactor was studied. Tthe result showed that the process of monochloramine decay under the effect of bio-film could be divided into two parts of fast decay stage in initial stage and subsequent stable decay stage. The decay process in stale stage could be described with zero-order kinetic model. The decay coefficient in stable decay stage has positive linear relationship with AOC concentration of water used for bio-film incubation. The higher monochloramine concentration in bulk phase could accelerate the decay of monochloramine with effect of bio-film and the effect appears distinct with higher count of microbiology on bio-film. Higher cut force could accelerate monochloramine decay under the effect of bio-film.Monochloramine decay under the effect of corrosion deposit accrete to the interior cast iron pipe wall were studied using self-designed pipe section reactor. The study result showed that monochloramine decay under the effect of corrosiondeposit could described by first order kinetic model; Higher monochloramine concentration in bulk phase could increase the decay coefficient indicate that the controlling factor of monochloramine decay under the effect of corrosion deposit is monochloramine mass transport from bulk phase to the surface of interior pipe wall. Low pH and low DO in bulk phase both can accelerate monochloramine decay under the effect of corrosion deposit and higher water velocity can also accelerate monochloramine decay.From the investigation to drinking water distribution system in field of Tanjin city, it is found that pipe wall including bio-film and corrosion deposit is most important factor causing monochloramine decay in drinking water distribution. The conclusion from the experiment in laboratory is the same to the one got from the investigation in field. The key point to control monochloramine decay in drinking water distribution systems is to control the corrosion of interior pipe wall...